Ambient light sensors (ALS) are widely used in many applications, for example, in mobile electronic devices such as cell phones and portable computers. Ambient light sensors allow display brightness to be automatically adjusted based on the light intensity in the environment. As a result, battery power consumed by the display is optimally managed and the user's viewing comfort is improved. One skilled in the art will recognize that there are many other applications for ambient light sensors.
A common type of light detecting devices used in the ambient light sensor is the silicon photodiode. Its simple structure renders silicon photodiode easy-to-use and a low-cost solution readily available from today's mainstream semiconductor manufacturing technologies such as complementary metal-oxide-semiconductor (CMOS) technology. CMOS technology allows silicon photodiode to be easily integrated on a same chip with analog and digital circuits required to perform the light sensing and control functions.
FIG. 1 illustrates the basic structure of an exemplary photodiode 100, where an N-type region 102 is shown inside a P-type layer (e.g., P-well, P-type substrate or P-type epitaxial layer) 104. The N-type region (e.g., N+ or N-well) is more heavily doped than the P-type layer and forms a PN junction with the P-type layer. To perform the light detecting function, PN junction is reverse-biased with a DC power supply 106.
Referring to FIG. 1, when light 108 is incident on a photodiode, electron-hole pairs 110 (also referred to as photocarriers, or carriers) are generated as a result of light absorption in silicon. Once generated, electrons and holes move toward cathode (N-type region) and anode (P-type layer), respectively. Carriers that do not recombine in silicon, for example, most of the carriers generated within and in the vicinity of the depletion layer 112 of the photodiode PN junction, flow out of the cathode and anode terminals and produce the photocurrent. This photocurrent is indicative of the intensity of the incident light 108.
While the silicon photodiode has a number of aforementioned benefits, a major problem is that its spectral response does not match that of the human eye. FIG. 2 shows the spectral response of the human eye 202, commonly known as the CIE photopic curve, and a spectral response 204 of an exemplary silicon photodiode. The human eye detects light in a narrow range of wavelengths, between 400 nanometers (nm) and 700 nm, in which the eye sensitivity peaks at around 550 nm. Notice in FIG. 2 that the spectral response of the exemplary silicon photodiode is much broader than that of the human eye, extending beyond 700 nm well into the infrared range. Because of this mismatch, the light intensity as detected by a silicon photodiode and as perceived by the human eye can differ significantly. The mismatch can be especially problematic for the light sources that emit a large amount of infrared such as incandescent lamps and the sun. The presence of an infrared source, such as a heater, near a silicon photodiode also interferes with and disrupt light sensing ability of the silicon photodiode.
To circumvent problems associated with the infrared sensitivity of silicon photodiode, conventional silicon light sensors employ multiple photodiodes and optical filters. FIG. 3 shows one such exemplary light sensor 300 found within the prior art, in this example, using two photodiodes, 302 and 304 and optical filters 306 and 308. The first photodiode 302 is covered with a green filter 306. Optical filters commonly used in light sensors are organic filters and do not cut off infrared. As such, visible light and a portion of infrared light pass through the green filter 306 and enter the first photodiode 302. The second photodiode 304 is covered with a green filter 306 and a red filter 308. As a result, the visible light is filtered out and only a portion of the infrared light enters the second photodiode 304. The photocurrent measured from the first photodiode 302 is indicative of the intensity of the visible light and a portion of infrared. The photocurrent measured from the second photodiode 304 is indicative of the intensity of a portion of infrared. By subtracting the photocurrent of the second photodiode from that of the first and applying a weighting factor between the two photocurrents, infrared component in the first photocurrent is eliminated and the resultant photocurrent represents the intensity of the visible light. One skilled in the art will recognize that there can be other ways to produce such an information regarding visible light intensity using multiple photodiodes and optical filters.
In conventional ambient light sensors found within the prior art, the need to accommodate multiple photodiodes in a light sensor increases die size, which in turn increases the die cost. Use of optical filters increases manufacturing cost and can be a reliability concern during operation at high temperatures or under long exposure to ultraviolet irradiation. Subtraction of one large infrared photocurrent from another can introduce large errors and lead to inaccurate results. What is desired is an ambient light sensor in which the photocurrent generation by infrared is suppressed so that the shortcomings of the conventional light sensors are mitigated.